JP2000273199A - Cellulose ester film, its production, protective film for polarizing plate, and polarizing plate prepared from the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for producing a cellulose ester film having few optical defects and excellent environmental suitability, which comprises preparing a cellulose ester solution, filtering the solution, and utilizing the cellulose ester solution remaining in the filtration process as the starting material. SOLUTION: This process is the one for producing a cellulose ester film by preparing a cellulose ester solution, filtering the solution, and purifying and drying the filtered solution, recovering the cellulose ester solution remaining in the filtration process, and adding 0.1-20 wt.% recovered solution to a starting solution to permit its reutilization, wherein the process is performed in a closed environment. The cellulose ester solution is prepared by mixing a cellulose ester with a poor solvent such as methanol to swell it, mixing the mixture with a good solvent such as methylene chloride under an elevated pressure at the boiling point of the solvent or higher to form a solution having a concentration of 10-30 wt.%. Poor and good solvents are used in a ratio of (10-30)/(70-95) by weight. The cellulose ester is desirably a cellulose triacetate having a degree of polymerization of 250-400 and a combined acetic acid content of 54-62.5 wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cellulose ester film, a method for producing the same, a protective film for a polarizing plate, and a polarizing plate using the same.

[0002]

2. Description of the Related Art Cellulose ester films are mainly used as supports for recording materials such as photographic materials. In recent years, it is also used for a polarizing plate used in a liquid crystal display device whose demand is increasing.

A polarizing plate is obtained by adsorbing and orienting iodine or a dichroic dye on polyvinyl alcohol to form a polarizing element, and covering this polarizing element with a protective film.

It is desirable that the material used for this protective film has a low optical anisotropy and a high transparency.

A cellulose ester film such as a cellulose triacetate film (triacetyl cellulose film) formed by a solution casting method has been used to satisfy these requirements.

However, it has been found that the conventional cellulose triacetate film has a bright spot foreign matter. This bright spot foreign matter is obtained by placing a cellulose triacetate film between two polarizers arranged in an orthogonal state (crossed Nicols), irradiating light from the outside of one polarizer, and using a microscope from the outside of the other polarizer. When observed, the light leaks from the foreign material portion, and the foreign material appears to shine like a star.

[0007]

In recent years, the development of technologies for improving the size and quality of liquid crystal display devices has been progressing, and the quality requirements for protective films for polarizing plates have become stricter. Therefore, a cellulose triacetate film in which many bright spot foreign substances are present is not preferred. This is because when used as a protective film for a polarizing plate, it causes light leakage and significantly deteriorates the quality of the polarizing plate.

On the other hand, when a cellulose triacetate film is produced by a solution casting method, the dope is generally filtered because there are undissolved substances in the dope. Since the cellulose ester solution has a high viscosity, a filter press having a large filtration area or a disk-shaped metal filter is used.

These filter media need to be replaced periodically because undissolved substances are captured and the pressure increases. At the time of replacement, it is necessary to once remove the cellulose triacetate solution in the filter.

Conventionally, the cellulose triacetate solution remaining in the filter is once extracted into a vat, and the solvent is evaporated and then discarded. When the solvent is extracted, the solvent evaporates, which is a problem in the working environment. In addition, since it is discharged as waste, it is a major environmental problem.

An object of the present invention is to provide a method for producing a cellulose ester film having less optical defects and excellent environmental suitability, a cellulose ester film produced by the method, a polarizing plate protective film and a polarizing plate using the same. It is in.

[0012]

The object of the present invention is attained by adopting one of the following constitutions.

[1] a step of preparing a cellulose ester solution, a step of filtering the solution, and
A method for producing a cellulose ester film, comprising: drying a cellulose ester solution remaining in a step of filtering the solution as a raw material in a method for producing a cellulose ester film through a drying step.

[2] a step of preparing a cellulose ester solution, a step of filtering the solution, and
The method for producing a cellulose ester film through a drying step, wherein the cellulose ester solution produced in the step of preparing the cellulose ester solution contains a cellulose ester solution remaining in the step of filtering the solution. A method for producing a cellulose ester film.

[3] The cellulose ester solution prepared in the step of preparing the cellulose ester solution contains 0.1 to 20% by weight of the cellulose solution remaining in the step of filtering the solution. [2] The method for producing a cellulose ester film according to [2].

[4] The operation until the solution remaining in the step of filtering the solution is recovered and added to the step of preparing a cellulose ester solution is performed in a closed environment [1]. The method for producing a cellulose ester film according to any one of [2] and [3].

[5] The cellulose ester solution remaining and recovered in the step of filtering the solution has a solid content concentration lower than that of the cellulose ester solution produced in the step of preparing the cellulose ester solution. The method for producing a cellulose ester film according to any one of [1] to [4].

[6] The cellulose ester is characterized in that the cellulose ester is cellulose triacetate.
[5] The method for producing a cellulose ester film according to any one of [5].

[7] A cellulose ester film produced by the method according to any one of [1] to [6].

[8] A polarizing plate protective film comprising the cellulose ester film of [7].

[0021]

[9] A polarizing plate obtained by laminating the cellulose ester film according to [8] and a polarizer.

The cause of the bright spot foreign matter is an insoluble matter contained in the dope. In the cellulose triacetate, hemicellulose such as mannose and xylose as impurities are contained.
It has been thought that it contains non-acetylated cellulose. It has been considered that when these are made into a cellulose solution, they do not completely dissolve but remain as undissolved substances, which have a bad effect as bright spot foreign matter.

As a countermeasure against bright spot foreign matter, it has been practiced to increase the filtration accuracy. However, when the filtration accuracy is increased, the filtration life is shortened, the frequency of replacement of the filtration material is increased, the workability is deteriorated, and the running cost is increased due to the increase in the amount of the filtration material, resulting in a significant inconvenience in film production. Therefore, it did not lead to providing a cellulose ester film having excellent workability and low production cost and a production method thereof.

The inventors of the present invention have been intensively studying the bright spot foreign matter, and have reused a solution of cellulose triacetate (hereinafter sometimes referred to as a dope) remaining in the filtration step as a raw material. It was found that the obtained cellulose triacetate film had significantly reduced bright spot foreign matter.

That is, although the cause of the bright spot foreign matter has not yet been determined, the remaining dope discharged when the filter medium is replaced in the step of filtering the dope was reused as a raw material. The cellulose triacetate film was found to have significantly reduced bright spot foreign matter. The present invention has been made based on this finding.

Further, by using the present invention, the amount of waste is reduced by reusing the remaining dope which has been conventionally discarded, and the cellulose triacetate film can be produced by an environmentally friendly production method.

That is, the above-mentioned problem is solved by a cellulose triacetate film obtained by reusing the dope remaining in the step of filtering the dope as a raw material.

Particularly, the dope to be reused is 0.1 to 0.1% by weight.
Preferably, the content is 20%, more preferably 0.2 to
It is better to contain 10%.

In particular, better results can be obtained if the solids concentration of the dope to be recycled is lower than the solids concentration of the dope produced in the step of preparing the dope. For this reason, it is preferable that the solvent used for washing the filtration step is further added to the dope remaining in the filtration step, mixed, and used as a dope to be reused.

When the dope remaining in the filtration step is recovered and reused, it can be carried out in a closed environment without causing the solvent to leak into the work place and without impairing the work environment.

Next, requirements and compounds according to the present invention will be described.

The cellulose ester solution according to the present invention comprises:
This is a solution in which a cellulose ester is dissolved in a solvent (solvent) and a plasticizer is usually added to the solution. Of course, other additives are often added as needed.

Examples of the cellulose ester according to the present invention include cellulose triacetate, cellulose diacetate, cellulose acetate butyrate, and cellulose acetate propionate.
Cellulose triacetate having a bound acetic acid content of 54 to 62.5% is preferable, and a bound acetic acid content of 58 to 6
2.5% is preferable because the base strength is strong. As the cellulose triacetate, either cellulose triacetate synthesized from cotton linter or cellulose triacetate synthesized from wood pulp can be used alone or in combination. If the releasability from the belt or drum becomes a problem, it is preferable to use a large amount of cellulose triacetate synthesized from cotton linter having good releasability from the belt or drum because the productivity is high. When a mixture of cellulose triacetate synthesized from wood pulp is used, the ratio of cellulose triacetate synthesized from cotton linter is preferably 40% by weight or more, and the effect of releasability becomes remarkable, and more preferably 60% by weight or more. ,
Most preferably, they are used alone.

The solvent used in the present invention is preferably a mixture of a good solvent and a poor solvent from the viewpoint of production efficiency.
The mixing ratio of the good solvent and the poor solvent is 70 to 95% by weight of the good solvent,
The content of the poor solvent is preferably 30 to 5% by weight. The concentration of the cellulose ester is preferably 10 to 30% by weight, and 18 to 2% by weight.
0% by weight is more preferred.

In the present invention, a good solvent and a poor solvent are defined as a good solvent that dissolves the cellulose ester used alone and a poor solvent that swells or does not dissolve alone. For this reason, a good solvent and a poor solvent change depending on the amount of bound acetic acid of the cellulose ester. For example, acetone is a good solvent when the amount of bound acetic acid is 55% and a poor solvent when the amount of bound acetic acid is 60%.

The good solvent used in the present invention includes organic halogen compounds such as methylene chloride and dioxolanes.

The poor solvent used in the present invention is preferably, for example, methanol, ethanol, n-butanol or cyclohexane.

As the method for dissolving the cellulose ester according to the present invention, a general method can be used, but a preferred method is to mix the cellulose ester with a poor solvent,
It is a method of wetting or swelling and further mixing with a good solvent. At this time, under pressure, the method of heating at a temperature not lower than the boiling point of the solvent at room temperature and the solvent does not boil and dissolving while stirring, in order to prevent the generation of a mass undissolved substance called gel or mamako, More preferred.

In addition to the cellulose ester and the solvent, necessary additives such as a plasticizer and an ultraviolet absorber may be mixed with the solvent in advance, dissolved or dispersed, and then added to the solvent before dissolving the cellulose ester. It may be added to the dope after dissolution.

In this case, it is preferable to concentrate and dissolve under high pressure. The type of pressurized container is not particularly limited, and it is sufficient if the container can withstand a predetermined pressure and can be heated and stirred under pressure. For the pressurized container, other instruments such as a pressure gauge and a thermometer are appropriately arranged.

Pressurization may be performed by a method of injecting an inert gas such as nitrogen gas or by increasing the vapor pressure of the solvent by heating.

The heating is preferably performed from the outside. For example, a jacket type is preferable because the temperature can be easily controlled.

The heating temperature after the addition of the solvent is preferably not lower than the boiling point of the solvent used and not in the range where the solvent does not boil. . The pressure is determined so that the solvent does not boil at the set temperature.

After the dissolution, the resin is taken out of the container with cooling, or is taken out of the container with a pump or the like, cooled with a heat exchanger or the like, and provided for film formation. The cooling temperature at this time may be cooled to room temperature, but it is more preferable to cool to a temperature 5 to 10 ° C. lower than the boiling point and perform casting at that temperature because the dope viscosity can be reduced.

In the present invention, a dope obtained by dissolving a cellulose ester in a solvent is cast on a support (casting step) and heated to remove a part of the solvent (drying step on the support). The film is peeled off from the support, and the peeled film is dried (film drying step) to obtain a cellulose ester film.

As the support in the casting step, a support in which a belt-shaped or drum-shaped stainless steel is mirror-finished is preferably used. The temperature of the support in the casting step can be cast in a general temperature range of 0 ° C. to a temperature lower than the boiling point of the solvent, but casting on the support at 0 to 30 ° C. causes gelation of the dope. It is preferable because the separation time can be increased, and it is more preferable to cast on a support at 5 to 15 ° C. The peeling limit time is the limit of the casting speed at which a transparent and flat good film can be continuously obtained.
The time during which the cast dope remains on the support. It is preferable that the peeling limit time is short because the productivity is excellent.

In the on-substrate drying step, the time required for casting the dope, gelling it once, and separating it from the casting is 10 minutes.
Assuming 0%, dope temperature within 4% within 30% of casting.
By setting the temperature to 0 to 70 ° C., the evaporation of the solvent is promoted, and the solvent can be peeled off from the support sooner, and the peeling strength is further increased.
It is more preferable to set the temperature to 70 ° C. This temperature is 20%
It is preferable to maintain the above, more preferably 40% or more. Drying on the support is preferably carried out with a residual solvent amount of 60 to 150%, because the peel strength from the support is reduced, and more preferably 80 to 120%. The temperature of the dope at the time of peeling is preferably 0 to 30 ° C., because the base strength at the time of peeling can be increased and the base can be prevented from breaking at the time of peeling, and more preferably 5 to 20 ° C.

In the film drying step, the film peeled from the support is further dried, and the residual solvent amount is reduced to 3% by weight.
Or less, preferably 0.5% by weight or less. In the film drying step, a method of drying while transporting a film by a roll suspension method or a pin tenter method is generally employed. For liquid crystal display members, it is preferable to dry while maintaining the width by a pin tenter method in order to improve dimensional stability. In particular, it is particularly preferable to maintain the width in a place where the amount of the residual solvent is large immediately after being peeled off from the support, since the effect of improving the dimensional stability is further exhibited. The means for drying the film is not particularly limited, and is generally performed using hot air, infrared rays, a heating roll, a microwave, or the like. It is preferable to use hot air in terms of simplicity. Drying temperature is 3 ~ in the range of 40 ~ 150 ° C.
It is preferable that the temperature is divided into five stages and the temperature is gradually increased, and it is more preferable to perform the process in the range of 80 to 140 ° C. in order to improve dimensional stability.

The thickness of the cellulose ester film according to the present invention is from 10 to 500 μm, preferably from 40 to 100 μm, particularly preferably from 40 to 85 μm for a liquid crystal display member.

Further, if necessary, fine particles such as silicon oxide may be added to the cellulose ester film according to the present invention as a matting agent. Fine particles such as silicon oxide are preferably surface-treated with an organic substance because the haze of the film can be reduced. Preferred organic substances for the surface treatment include halosilanes, alkoxysilanes, silazanes, siloxanes and the like. The larger the average diameter of the fine particles, the greater the matting effect, and the smaller the average diameter, the better the transparency. Therefore, the average diameter of the primary particles of the fine particles is preferably 5 to 50 nm, more preferably 7-14.
nm.

As fine particles of silicon oxide, AEROSIL 200, 300, R972 manufactured by Aerosil Co., Ltd.
R974, R202, R812, OX50, TT600
And the like, preferably AEROSILR972,
R974, R202, R812 and the like.

An ultraviolet absorber can be used in the cellulose ester film of the present invention. As the ultraviolet absorber, those which are excellent in the ability to absorb ultraviolet light having a wavelength of 370 nm or less from the viewpoint of preventing deterioration of the liquid crystal and absorb as little as possible visible light having a wavelength of 400 nm or more from the viewpoint of good liquid crystal display properties are preferable. Used. Commonly used compounds include, for example, oxybenzophenone compounds, benzotriazole compounds, salicylic acid ester compounds, benzophenone compounds, cyanoacrylate compounds,
Nickel complex compounds, inorganic powders and the like can be mentioned.

As the benzotriazole-based ultraviolet absorber, a compound represented by the following general formula [1] is preferably used.

[0054]

Embedded image

In the formula, R ₁ , R ₂ , R ₃ , R ₄ and R ₅ may be the same or different and include a hydrogen atom, a halogen atom, a nitro group, a hydroxyl group, an alkyl group, an alkenyl group, an aryl group, Represents an alkoxy group, an acyloxy group, an aryloxy group, an alkylthio group, an arylthio group, a mono- or dialkylamino group, an acylamino group or a 5- to 6-membered heterocyclic group, wherein R ₄ and R ₅ are closed to form a 5- to 6-membered group; It may form a carbocycle.

As the inorganic powder, ultrafine titanium oxide,
Examples include ultrafine zinc oxide, ultrafine iron oxide, ultrafine cerium oxide, and silica-cerium oxide coated pigment.

The ultraviolet absorber preferably used in the present invention is preferably a benzotriazole-based ultraviolet absorber or a benzophenone-based ultraviolet absorber which has high transparency and has an excellent effect of preventing deterioration of a polarizing plate or a liquid crystal element. Particularly preferred is a benzotriazole-based ultraviolet absorber having a smaller content.

The method of adding the ultraviolet absorber may be such that the ultraviolet absorber is dissolved in an organic solvent such as alcohol, methylene chloride or dioxolane and then added to the dope, or may be added directly to the dope composition. Those which do not dissolve in an organic solvent such as inorganic powders are dispersed in an organic solvent and a cellulose ester using a dissolver or a sand mill and then added to the dope.

The amount of the ultraviolet absorbent used in the present invention is 100% in terms of transparency and ultraviolet absorbing ability.
The amount is preferably 0.2 to 4 g, more preferably 0.5 to 2 g, based on g.

The method for producing the polarizing plate according to the present invention is not particularly limited, and it can be produced by a general method. For example, there is a method in which a cellulose triester film is alkali-treated, and is immersed and stretched in an iodine solution, and bonded to both surfaces of a polarizing film using a completely saponified polyvinyl alcohol aqueous solution. Instead of the alkali treatment, a method for increasing the adhesion as described in JP-A-6-94915 and JP-A-6-118232 may be used.

The cellulose ester film of the present invention preferably contains a plasticizer. There is no particular limitation on the plasticizer that can be used, but in the case of phosphate esters, triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl phosphate, etc. In the phthalate ester system, diethyl phthalate,
In the glycolic acid ester system such as dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, triacetin, tributyrin, butyl phthalyl butyl glycolate, ethyl phthalyl ethyl glycolate,
It is preferable to use methylphthalylethyl glycolate, butylphthalylbutyl glycolate or the like alone or in combination. 50% by weight of phosphate plasticizer
The following are preferable because they hardly cause hydrolysis of the cellulose ester film and are excellent in durability. It is more preferable that the ratio of the phosphate plasticizer is smaller, and it is particularly preferable to use only the phthalate or glycolate plasticizer.

As the plasticizer preferably used in the present invention, a plasticizer having a freezing point of 20 ° C. or less is preferable,
The temperature is not particularly limited as long as it is not more than ° C, and can be selected from the above plasticizers. For example, tricresyl phosphate,
Examples thereof include cresyl diphenyl phosphate, tributyl phosphate, diethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, triacetin, and ethyl phthalyl ethyl glycolate. It is preferable to use these plasticizers alone or in combination.

The freezing point in the present invention is defined as the true freezing point described in the Encyclopedia of Chemistry published by Kyoritsu Shuppan Co., Ltd.

The amount of these plasticizers to be used is 1 to 1 with respect to cellulose ester in terms of film performance, processability and the like.
15% by weight is preferred. For a liquid crystal display member, 1 to 10% by weight is more preferable from the viewpoint of dimensional stability, and 3 to 10% by weight.
Even more preferred is 7% by weight.

The content of the plasticizer having a freezing point of 20 ° C. or less with respect to the cellulose ester is preferably 1% by weight or more and 10% by weight or less, more preferably 3% by weight or more and 7% by weight or less. It is preferable that the ratio of the plasticizer having a freezing point of 20 ° C. or lower in the total plasticizer is large because the flexibility of the cellulose ester film is improved and the processability is excellent.
Most preferably, all of the plasticizer is a plasticizer having a freezing point of 20 ° C. or lower.

It is preferable to use a plasticizer having a freezing point of 20 ° C. or lower in an amount of 1% by weight based on the cellulose ester, since the flexibility of the cellulose ester film is improved and the processability is excellent. Use of a plasticizer having a temperature of 14 ° C. or lower is preferred because the processability is further improved.

The workability refers to a process of slitting or punching a base film or a liquid crystal display member. If the workability is poor, the cut surface becomes saw-toothed and chips are generated. Is not preferred.

The liquid crystal display member is a member used in a liquid crystal display device, and includes, for example, a polarizing plate, a protective film for a polarizing plate, a retardation plate, a reflecting plate, a viewing angle improving film, an antiglare film, and a non-reflective film. Films, antistatic films and the like.

Among them, it is more preferable to apply the present invention to a polarizing plate and a protective film for a polarizing plate, which have strict requirements for dimensional stability.

[0070]

EXAMPLES Next, the embodiments of the present invention will be specifically described with reference to examples, but the embodiments of the present invention are not limited to the description.

Comparative Example 1 Preparation of Dope Dope Formulation A Triacetylcellulose (degree of acetylation: 61.0%) 100 parts by weight 2- (2′-hydroxy-3 ′, 5′-di-t-butyl) Phenyl) benzotriazole 2 parts by weight Ethylphthalylethyl glycolate 5 parts by weight Methylene chloride 475 parts by weight Ethanol 50 parts by weight The above composition was placed in a closed vessel and completely dissolved with heating and stirring.

(Preparation of Sample) The above dope was filtered using a filter press, and then uniformly cast on a stainless steel band support at a dope temperature of 33 ° C. and a width of 1500 mm using a belt casting apparatus.

After evaporating the solvent until it could be peeled off on the stainless band support, it was peeled off from the stainless band support.

The peeled triacetyl cellulose film was passed through a drying zone using a number of rolls, and
The slit was cut to a width of mm to prepare a triacetyl cellulose film having a thickness of 80 μm.

After producing the film for 1 hour, the production of the film was stopped, the dope in the filter press was discharged into the vat by introducing compressed air, the filter press was disassembled, and the filter paper was replaced.

At that time, the concentration of methylene chloride in the air was measured using a predetermined detector tube.

The doped dope was placed in a steam drying box and
After drying for 4 hours to evaporate the solvent, the weight was measured and discarded.

After replacing the filter paper, the dope was prepared as described above,
It was cast and dried to obtain a sample of a cellulose triacetate film.

(Evaluation of Sample) The number of bright spot foreign matters was measured for the cellulose triacetate film obtained as described above.

For the bright spot foreign matter, the cellulose triacetate film is placed between two polarizing plates arranged in an orthogonal state (crossed Nicols), and light is irradiated from the outside of one of the polarizing plates.
The number of foreign matter (bright spot foreign matter) that appeared white out per 25 mm ² under the microscope from the outside of the other polarizing plate was measured at 100 locations, and the average value was evaluated.

At this time, the conditions of the microscope were a transmission light source at a magnification of 30 times. The smaller the number of bright spot foreign matters, the better.

Example 1 A dope was prepared, cast and dried in the same manner as in Comparative Example 1 to produce a triacetyl cellulose film having a thickness of 80 μm.

After the film was prepared for 1 hour, when the dope in the closed container became empty, the film preparation was stopped, the dope in the filter press was drawn out into the vat by introducing compressed air, and the filter press was disassembled. I changed the filter paper.

The methylene chloride concentration in the air at that time was measured using a predetermined detector tube.

The extracted dope was returned to the closed container for preparing the dope.

The dope was prepared so that the weight ratio of the returned dope to the newly prepared dope was 1:99, and a triacetyl cellulose film having a thickness of 80 μm was prepared in the same manner.

The sample of the obtained triacetylcellulose film was evaluated for bright spot foreign matter.

Example 2 A dope was prepared, cast and dried in the same manner as in Comparative Example 1 to produce a triacetyl cellulose film having a thickness of 80 μm.

After the film was prepared for 1 hour, when the dope in the closed container was exhausted, the dope in the filter press was drawn out through a closed pipe using compressed air into a closed container, and the filter press was disassembled. The filter paper was replaced.

The methylene chloride concentration in the air at that time was measured using a detector tube.

Next, the dope extracted before the preparation of the dope was returned to the closed container for preparing the dope through a closed pipe.

Next, a dope was prepared so that the weight ratio of the returned dope to the newly prepared dope was 1:99 by weight, and a sample of a triacetyl cellulose film having a thickness of 80 μm was prepared in the same manner.

The samples of the obtained triacetylcellulose film were evaluated for bright spot foreign matter.

Example 3 A dope was prepared, cast and dried in the same manner as in Comparative Example 1 to produce a triacetyl cellulose film having a thickness of 80 μm.

After the film was prepared for 1 hour, when the dope in the closed container was exhausted, the dope in the filter press was passed through a closed pipe using compressed air, extracted into the closed container, and the filter press was disassembled. The filter paper was replaced.

The concentration of methylene chloride in the air at that time was measured using a detector tube.

Next, the dope withdrawn before the preparation of the dope was returned to the closed container for preparing the dope through a closed pipe.

Next, a dope was prepared such that the weight ratio of the returned dope and the newly prepared dope was 10:90 by weight.

In the same manner, a sample of a triacetyl cellulose film having a thickness of 80 μm was prepared.

The sample of the obtained triacetylcellulose film was evaluated for bright spot foreign matter.

Example 4 A dope was prepared, cast and dried in the same manner as in Comparative Example 1 to produce a triacetyl cellulose film having a thickness of 80 μm.

After the film was produced for 1 hour, when the dope in the closed container was exhausted, the dope in the filter press was drawn out through a closed pipe using compressed air into the closed container.

The methylene chloride concentration in the air at that time was measured using a detector tube.

Next, a solvent having the same solvent composition as that used in the preparation of the dope was passed through a filter press using a pump to wash the solvent.

After washing, the solvent and the dope were placed in a container from which the solvent had been previously removed, and mixed at a weight ratio of 50:50.

Next, before the dope preparation, the extracted dope was returned to the closed container for dope preparation through a closed pipe, and the weight ratio of the dope to the newly prepared dope was 1:99 by weight. Was prepared.

Otherwise, the thickness was 80 μm in the same manner as in Comparative Example 1.
Was prepared.

The sample of the obtained triacetylcellulose film was evaluated for bright spot foreign matter.

Example 5 A dope was prepared, cast and dried in the same manner as in Comparative Example 1 to produce a triacetyl cellulose film having a thickness of 80 μm.

After the film was prepared for 1 hour, when the dope in the closed container was exhausted, the dope in the filter press was drawn out through a closed pipe using compressed air into a closed container.

The methylene chloride concentration in the air at that time was measured using a detector tube.

Next, a solvent having the same solvent composition as that used in the preparation of the dope was passed through a filter press using a pump to wash the solvent.

The solvent and the dope were placed in a container containing the solution from which the solvent had been removed after the washing so as to have a weight ratio of 70:30, and mixed.

Next, before performing the dope preparation, the dope mixed with the solvent was returned to the closed container for preparing the dope through a closed pipe.

The dope was prepared such that the weight ratio of the returned dope and the newly prepared dope was 1:99 by weight.

In the same manner, a sample of a triacetyl cellulose film having a thickness of 80 μm was prepared.

The samples of the obtained triacetylcellulose film were evaluated for bright spot foreign matter.

Example 6 A dope was prepared, cast and dried in the same manner as in Comparative Example 1 to produce a triacetyl cellulose film having a thickness of 80 μm.

After the film was prepared for 1 hour, when the dope in the closed container was exhausted, the film was taken out of the closed container through a closed pipe by using compressed air of the dope in the filter press.

At that time, the concentration of methylene chloride in the air was measured using a detector tube.

Next, a solvent having the same solvent composition as that used in the preparation of the dope was passed through a filter press using a pump to wash the solvent.

The solvent and the dope were placed in a container containing the solution from which the solvent had been removed after the washing had been removed so as to have a weight ratio of 70:30, and mixed.

Next, before preparing the dope, the dope mixed with the solvent was returned to the closed container for preparing the dope through a closed pipe.

The dope is prepared such that the weight ratio of the returned dope and the newly prepared dope is 10:90 by weight. Using this, a sample of a triacetyl cellulose film having a thickness of 80 μm was prepared in the same manner as in Comparative Example 1.

A sample of the obtained cellulose triacetate film was evaluated for bright spot foreign matter.

The results obtained are summarized in Table 1 below.

[0127]

[Table 1]

It can be seen that Examples 1 to 6 of the present invention have less bright spot foreign matter than Comparative Example 1 of the present invention.

[0129]

According to the present invention, there are provided a method for producing a cellulose ester film having less optical defects and excellent environmental suitability, a cellulose ester film produced by the method, a polarizing plate protective film and a polarizing plate using the same. I can do it.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 2H049 BA02 BB33 BB51 BC09 BC22 4F071 AA01B AA09 AA09B AH12 BC01 BC02 4F100 AJ06A AJ06K AR00B BA02 GB90 JL16 JN10 JN10B

Claims (9)

[Claims] 1. A method for producing a cellulose ester film, which comprises a step of preparing a cellulose ester solution, a step of filtering the solution, and a step of forming and drying the solution after filtration, remaining in the step of filtering the solution. A method for producing a cellulose ester film, comprising reusing a cellulose ester solution as a raw material. 2. A process for preparing a cellulose ester solution, comprising the steps of: preparing a cellulose ester solution; filtering the solution; and film-forming and drying the solution after filtration. A method for producing a cellulose ester film, characterized in that the cellulose ester solution produced comprises a cellulose ester solution remaining in the step of filtering the solution. 3. The cellulose ester solution prepared in the step of preparing the cellulose ester solution contains 0.1 to 20% by weight of the cellulose solution remaining in the step of filtering the solution. Claim 2
A method for producing the cellulose ester film according to the above. 4. The operation of recovering a solution remaining in the step of filtering the solution and adding the solution to a step of preparing a cellulose ester solution is performed in a closed environment. 4. The method for producing a cellulose ester film according to any one of items 3 to 3. 5. The cellulose ester solution remaining and recovered in the step of filtering the solution, wherein the solid content of the cellulose ester solution is lower than that of the cellulose ester solution produced in the step of preparing the cellulose ester solution. The method for producing a cellulose ester film according to any one of claims 1 to 4. 6. The method for producing a cellulose ester film according to claim 1, wherein the cellulose ester is cellulose triacetate. 7. A cellulose ester film produced by the method according to any one of claims 1 to 6. 8. A polarizing plate protective film comprising the cellulose ester film according to claim 7. 9. A polarizing plate comprising the cellulose ester film according to claim 8 and a polarizer.